The triple quadrupole mass spectrometer is a well-known and widely used instrument for targeted analysis of complex mixtures, using molecular ion sources such as electrospray, atmospheric-pressure chemical ionization and others. In these instruments, precursor ions of a specific range of mass-to-charge (m/z) ratios are selected by one quadrupole analyzer (Q1), fragmented in a gas-filled collision cell (Q2) and then one or more particular fragments are selected by a second quadrupole analyzer (Q3). This allows filtering out only desired precursor ions and corresponding fragment ions of interest. Thus, a robust, quantitative method for targeted analysis is provided, where the targets for analysis are known but are present at very low levels comparing to other analytes.
It is also known that such instruments can be successfully applied to elemental analysis, which may use a range of ion sources, including: Inductively Coupled Plasma (ICP); glow discharge (GD); Microwave Induced Plasma (MIP); and others. A triple-quadrupole ICP mass spectrometry system has several advantages compared to those equipped with only one “full resolution” quadrupole (resolution depends on mass, but generally in the range of about 300 with peak widths in the range of 0.7-0.8 amu) and a collision or reaction cell, which might therefore be termed a dual-quadrupole device. In the triple-quadrupole, the quadrupole located upstream the collision or reaction cell allows selection of a limited set of ions, according to their m/z ratio, to undergo reactions inside the collision/reaction cell.
In one approach, the collision cell (Q2) operates as a reaction cell, filled with reactive gases such as oxygen (O2) or ammonia (NH3). Alternatively, the collisional cell (Q2) can be used in collision mode with an inert gas (such as He), other reactive gases (for example, H) or mixtures (H+He, for instance).
Some implementations based on these principles may react the interfering ions to another mass (or achieve this by collision), while the desired ions are left alone and can therefore be selected and detected in the Q3 analyzer. Examples of this are detailed in U.S. Pat. Nos. 7,202,470, 7,230,232 and 7,339,163. Alternatively, the desired ions may be reacted or collided to another mass, while the interfering ions are left alone. The Q3 analyzer may then be used to select and detect the desired ions at a different mass (a higher mass in the case of reaction) than the Q1 analyzer. Examples of such configurations are described in “Some Current Perspectives on ICP-MS,” D. J. Douglas, Canad. J. Spectrosc., V. 34, No. 2, 1989, U.S. Pat. No. 6,875,618, GB-2391383, WO-01/01446 and U.S. Pat. No. 8,610,053.
When the Q2 cell is operated in reaction mode, different element or adduct ions may react with the reaction cell gases at vastly different rates. Hence, if isotopes or adducts of different elements appear in the same mass window after being selected by Q1, the desired element ions could be further transformed in Q2, for instance into adduct products ([A+O]+ or [A+NH3]+, etc.), while the isotopes or adducts of interfering elements will typically remain at the same m/z ratio. In principle, the second quadrupole analyzer (Q3) can then select a product of interest and thus provides interference-free output from the detector.
However, achieving such reduced interference demands a significant complication of the instrument layout in comparison with a traditional single-quadrupole analyzer. It is suggested in U.S. Pat. No. 8,610,053 that this is caused by the necessity to add an analytical-quality Q1, requiring tight vacuum conditions to operate with unit mass resolution. Such an instrument is complex and expensive. It would be desirable to use a lower-resolution Q1, having lower vacuum requirements, but without causing increased interference or reducing abundance sensitivity.